Cross-Sections and Transects¶
Transect: A profile line running from inland across the shoreline to the limit of wave action
Velocity Zone (V Zone): Areas with wave heights greater than 3 feet; characterized by higher velocities due to wave action
Wave Height Zones:
| Wave Height | Zone | Description | |-------------|------|-------------| | Greater than 3 feet | V Zone | High velocity zone with strong wave action | | Less than 3 feet | A Zone (Coastal) | Moderate wave action; sometimes called Coastal A zone or LIMWA (Limit of Moderate Wave Action) |
Wave Run-up¶
- The vertical extent waves travel up the shore beyond the still water line
- Varies based on:
- Shoreline slope and shape
- Storm characteristics and direction
- Astronomical tides
- Water depth offshore
- Important for flood risk assessment and construction considerations
Roughness and Flow Velocity¶
Manning's Roughness Coefficient: Scale used to estimate flow velocity based on surface roughness
Critical for calculating water velocity in channels and floodways
Roughness varies with surface types:
| Surface Type | Roughness Impact | |-------------|------------------| | Tall Grass | Increased roughness, slows flow | | Concrete | Low roughness, faster flow | | Regular Grass | Moderate roughness |
Floodway and Profile Details¶
- Floodways are shown on maps but usually absent on FIS
- Profiles show elevations along a river or stream, including:
- Bridge crossings (depicted as I-beams) with bottom and top elevations
- Flood profiles are used to determine Base Flood Elevations (BFE)
Fundamentals of Flood Insurance Rate Maps (FIRMs)¶
Community and NFIP Identification¶
- FIRMs often cover multiple communities within a county
- Each community has a unique NFIP identification number
- Map identification includes:
- Community name
- County name
- NFIP ID number
- Important for documentation like elevation certificates and regulatory compliance
Map Revision Letters¶
- Each community's map revision is indicated by a letter suffix (e.g., A, B, C, D)
- Letter corresponds to revision number for that community, not the entire map
- Always check the revision letter for the latest version
- The word "revision" is not explicitly written; the letter alone signifies it
Effective Date¶
- Displayed on the front page of the map
- Using the most recent effective map is mandatory under NFIP rules
- Communities not using the latest map may violate federal regulations
Legend and Map Symbols¶
Legend: Essential for interpreting the map, though students don't need to memorize it
Key symbols include:
| Symbol | Meaning | |--------|---------| | Gray box with diagonal lines | Floodway area | | Shaded areas with hole | Zone X shaded (areas of minimal flood hazard) | | Area labeled "X" (unshaded) | Zone X unshaded: areas outside flood hazard | | B Zone | Areas with flooding but less severe than A or V zones | | D Zone | Areas with possible but undetermined flood hazard |
Profile Interpretation and Applications¶
Profiles help determine flood elevations at specific points like road crossings
Elevation lines (500-year, 100-year, 10-year flood elevations) are shown on profiles
Bridge crossings are precisely drawn to scale with bottom and top elevations
Profiles allow interpolation of exact flood elevations between known points
Coastal Barrier Resource Areas (CBRA) and Otherwise Protected Areas (OPA)¶
Designated coastal zones protected from development to conserve resources and reduce federal flood insurance exposure
Effective dates: 1983, 1990, 1991 (different areas)
Buildings constructed or substantially improved after these dates are not eligible for federal flood insurance
Marked on flood maps by diagonal crosshatching or solid diagonal lines
Different types of hatched areas correspond to different effective designation years
Practical Implications¶
Existing buildings built before designation dates may have flood insurance
Substantially damaged buildings rebuilt after the effective date cannot get flood insurance
Communities may allow construction permits, but federal flood insurance won't be available
This affects financing, as federally-backed loans usually require flood insurance
Owners may need private insurance or self-insurance
Special Map Features¶
Cross Sections¶
Slices or profiles of the floodplain used to analyze river/stream flood characteristics
Marked by hexagons on maps and labeled with letters (A, B, C) and numbers
Direction usually follows river flow downstream, but not always
Check the stationing distance (reference point) on the map or profile
Do not assume cross sections always increase in distance downstream without verification
Map Scale and Measurement¶
- Scales vary between maps (e.g., 500 feet, 1000 feet)
- Use a piece of paper to mark distances for rough measurements
- Some maps may not be to scale
- Always verify scale on map legend before making distance estimates
Finding Base Flood Elevation (BFE) Step-by-Step¶
Locate the River on the Map
- Identify the specific river
- Don't confuse with other rivers on the map
Find the Correct Cross Section
- Cross sections marked with hexagons and letters
- Verify you're on the correct river
Check the Floodway on the Map
- Designated by diagonal lines
- Base Flood Elevation in the floodway is your target
Read the Elevation on the FIRM
- Locate elevation numbers along the river
- Example: Elevation near cross section A might be approximately 10 ft
Consult the Profile if Needed
- Profiles show flood elevations vs. cross sections
- Use profile lines for 10-, 50-, 100-, and 500-year floods
Use the Floodway Data Table for Regulatory Numbers
- For BFE in the floodway, the data table is authoritative
- The profile may not always give the regulatory floodway elevation accurately
Common Pitfalls & Reminders¶
- Confusing rivers with same cross section letters: Always verify the river name
- Ignoring "floodway" in questions: Read carefully; "floodway" specifies using the floodway data table
- Misreading elevation lines on profiles: Identify the correct line style for the 100-year flood
- Assuming the first cross section with a letter is correct: Trace back to the correct river
Map Changes and Regulatory Considerations¶
Letters of Map Change (LOMC)¶
- LOMA (Letter of Map Amendment): Updates boundaries using better topography; doesn't change BFEs
- LOMR (Letter of Map Revision): Revises BFEs and boundaries due to physical changes
- CLOMR (Conditional Letter of Map Revision): Pre-approval before construction
- LOMR-F (Letter of Map Revision Based on Fill): Revises maps after fill is placed
Authority and Environmental Compliance¶
- Only FEMA can officially change flood maps
- Local governments can request changes but lack authority to make them
- Changes must consider the
Endangered Species Actand other environmental regulations - Development impacting protected species may be denied despite meeting floodplain criteria
Key Vocabulary¶
- Floodway: Channel and adjacent lands that must remain free of encroachment for base flood discharge
- Base Flood Elevation (BFE): Computed elevation of floodwater during the base (100-year) flood
- FIRM: Official map showing flood hazard areas and floodway boundaries
- Profile: Graphical representation of flood elevations along a river or stream
- Culvert: Man-made structure allowing water flow under a road or obstruction
Important Reminders¶
- Always check questions carefully to determine which source to use (profile, floodway table, or map)
- Interpret elevation data from profiles by reading the scale carefully
- Understand the difference between floodplain and floodway
- Know how to locate floodway limits on the FIRM map